1. Field of the Invention
The present invention relates to a system for measuring the direction in which a magnetometer is to be set, without use of any compass such as a magnetic compass, gyrocompass, etc.
2. Description of the Prior Art
In a conventional automatic steering gear for automatically controlling the steerage of a ship, a controller (a magnetometer) 3 having a course setting knob 2 is superposed on a magnetic compass 1 as illustrated by a structural block diagram in FIG. 1 and operated as will be described afterward. Denoted by 4 is a source of AC voltage. An output corresponding to the operation of the course setting knob 2 is obtained in a magnetism detecting circuit 5 and is supplied to a pair of terminals of a bridge circuit 6. FIG. 2 is a schematic view of a magnetometer which has excitation AC winding 11 and output winding 12. When a magnetic core 20 and the aforementioned windings are set in position as illustrated, the output winding has a waveform shown by FIG. 3B. Shown by FIG. 3A is an excitation AC waveform. The waveform FIG. 3B stands for the second high frequency harmonic of the excitation AC waveform and satisfies the relationship of V.sub.1 =V.sub.2. When the knob 2 of the controller is rotated to shift the windings in position relative to a magnetic needle, the output winding 12 has a waveform having the relationship of V.sub.1 &gt;V.sub.2 as shown by FIG. 3C. When the knob 2 is reversely rotated, the waveform has the relationship of V.sub.1 &lt;V.sub.2 as shown by FIG. 3D. This voltage, while the magnetism detecting circuit 5 regulates the angle of rotation of the knob and the variation in voltage, is supplied to one side of the bridge circuit 6. To another side of the bridge circuit is supplied an output from a follow-up potentiometer 10. The difference between the two outputs is output from the bridge circuit 6 and amplified in an amplifier 7. The amplified value causes a steering engine 8 to be driven and then a rudder 9 is allowed to move. The movement of the rudder 9 serves to actuate the follow-up potentiometer 10 and cause an input to the amplifier 7 to become zero. For this reason, the rudder 9 is set in a direction in which the knob 2 is rotated. As a result, the direction in which a ship advances relative to the magnetic needle 20 is to be recognized. A magnetic compass, when attached to a hull, gives rise to deviations due to iron structures on the hull and, in adopting automatic steering, sometimes causes a ship to advance meanderingly. A gyrocompass is used more frequently than a magnetic compass because the former has a better performance than the latter. However, a gyrocompass is disadvantageous in that a large amount of electrical power and a high rotation speed are required and consequently it becomes large in size.